Nanocomposites are materials containing two or more chemically dissimilar phases in which at least one of the phases has a nanoscale dimension. Nanocomposites consisting of exfoliated clay lamellae dispersed in an organic polymer matrix exhibit enhanced physical properties relative to virgin polymer, or to conventional macro- or microcomposites containing other inorganic fillers (e.g., glass fiber, talc, mica, carbon black) [1]. The enhancements may include improved tensile and flexural properties, increased storage modulus, increased heat distortion temperature, decreased flammability [32], decreased gas permeability, reduced visual defects and improved optical transparency [2].
The clay filler achieves these improvements at very low clay loadings (≦5 wt %), thus the material retains desirable polymer properties such as light weight, low cost, solution/melt processability and recyclability. Uses for these nanocomposite materials include molded automotive and appliance components (such as body panels, under hood components, electrical/electronic parts and insulation, power tool housings) and furniture (such as seat components, consoles), medical tubing, abrasion and chemical resistant coatings, food packaging materials (such as transparent stretch films) and barrier layers for beverage bottles.
Clays such as kaolinite, hectorite and montmorillonite (MMT) have been investigated as mechanical supports for single-site ethylene polymerization catalysts [3]. Usually the support is also treated with an organoaluminum co-catalyst, such as a trialkylaluminum or an alkylaluminoxane, which serves to remove adsorbed water and passivate the clay surface. It has also been suggested that alkylaluminum compounds can cause delamination of kaolinite [4]. In general, the catalyst is adsorbed onto the co-catalyst-modified clay, where it is activated in situ by the co-catalyst surface layer [5], [6]. Olefin uptake by the supported catalyst results in controlled particle growth, which is a desirable behavior in polymerization reactor engineering.
Supporting metallocene catalysts on clays results in modest activity for ethylene polymerization [7], even in the absence of alkylaluminum co-catalysts [8]. However these catalyst systems do not generate high quality nanocomposites; the polyethylene they produce contains small clumps of unexfoliated clay.
The desirable physical properties of nanocomposites are observed only when clay sheets are highly dispersed in the polyolefin matrix. The difficulty in making exfoliated clay-polyolefin nanocomposites originates in the immiscibility of strongly associated hydrophilic clay sheets and hydrophobic polyolefin chains. In many varieties of clay, clay layers are negatively charged due to isomorphic substitution of framework ions, generally cations. Interlayer cations provide charge compensation and promote strong interlayer adhesion, which simple mixing with a polyolefin cannot effectively disrupt.
One strategy to make the components of the nanocomposite compatible is to render the clay hydrophobic, by replacing the interlayer ions with surfactants such as long chain alkylammonium, imidazolium or alkylphosphonium cations (typically C18). This procedure generates an organically-modified layered silicate (OMLS). Methods employing an OMLS in the preparation of polyolefin nanocomposites include:                In situ intercalative polymerization, in which a catalyst adsorbed onto the OMLS, causes spontaneous delamination upon addition of monomer. This strategy has been successfully applied to propylene polymerization using a zirconocene catalyst supported on methylaluminoxane (MAO)-treated OMLS [9], and to ethylene polymerization using a Brookhart Pd catalyst supported on OMLS [10]. The Ziegler catalyst TiCl4, grafted onto a hydroxyl-containing surfactant intercalated into MMT, was used for in situ polymerization of ethylene upon activation with triethylaluminum [11]. Silica or titania nanoparticles synthesized in the interlayer spaces of an OMLS by a sol-gel method were treated with an alkylaluminum and a metallocene to create a catalyst system for in situ polymerization [12]. In situ polymerization filling was achieved using MAO-treated clay and metallocene or constrained geometry catalysts with [13] and even without [14], [15] surfactant modification of the clay. In the absence of surfactant, the clay was swollen using an organic solvent.        Solution intercalation, in which high density polyethylene (HDPE) dissolved in a hot xylene/benzonitrile mixture is stirred with dispersed OMLS [16];        Melt intercalation, in which the OMLS is annealed with polymer above the softening point of the latter, either statically or under shear. Since mixing is driven by interactions between the polymer and the clay, this method typically requires a compatibilizer consisting of polymers or oligomers modified with polar sidechains or endgroups. For example, nanocomposite formation was achieved by melt intercalation of propylene oligomers with telechelic OH groups, followed by melt-mixing with unmodified PP [17]. Melt blending of PP and OMLS was achieved using a twin screw extruder in the presence of maleated PP (i.e., functionalized with maleic anhydride side chains, PP-g-MA) as the compatibilizer [18, 19, 20, 21]. A similar strategy was used to make nanocomposites by melt blending of PE-g-MA [22], [23] or EPR-g-MA [23] with OMLS. A semifluorinated surfactant was used to create an OMLS with weaker clay-surfactant interactions and a greater propensity to intercalate unmodified PP [24]. A method involving functionalized surfactants which react to form chemical bonds with the maleated compatibilizer has been described [25]. Direct melt intercalation of ammonium-functionalized polypropylene chains into unmodified MMT was achieved, presumably by direct cation exchange, without intermediate functionalization of the clay with surfactant [26].        
Recently, the formation of nanocomposites with unmodified clay was achieved by making the polyolefin component more hydrophilic. In the presence of the surfactant cetyltrimethylammonium bromide, micelles containing polystyrene were formed and adsorbed from solution onto dispersed clay [27].
Also recently, nanocomposite materials have been produced by adding an olefin to a suspension of acid-treated layered silicate treated with a solution of a metallocene polymerization catalyst, causing olefin polymerization to form the nanocomposite polymer [28]. Although described in broad encompassing terms, the specific preparations described by the reference all require the use of a tripropylaluminum co-catalyst added to the slurry formed by mixing 4-tetradecylanilinium-exchanged or HCl-treated clay to dry toluene.
A flame retardant is a material that exhibits either a delay in the start, or a decrease in the rate of propagation, of a fire [30, 31]. Organic polymers can be made flame retardant by incorporating a large quantity (ca. 50 wt %) of an inorganic (e.g., Mg(OH)2) or organic (e.g., brominated polystyrene) filler. Flame retardant properties may be obtained at much lower filler content with nanocomposites. Potential uses for flame retardant nanocomposite materials include molded furniture, automotive parts (such as body panels, under hood components) and appliance components (such as electrical/electronic parts, power tool housings).
The first report of improved thermal stability in a polymer-clay composite involved a polymethylmethacrylate (PMMA)-montmorillonite (MMT) clay system. At 10 wt % clay loading, this material exhibits an increase of 40-50° C. in its thermal decomposition temperature relative to pure PMMA [33]. A nanocomposite prepared by sonication of silanol-terminated polydimethylsiloxane (PDMS) with montmorillonite (10 wt %) decompose at a temperature 140° C. higher than pure PDMS [34]. An increase in the decomposition temperature was observed upon melt intercalation of aliphatic polyimide (PEI-10) into clay [35]. An increase in the thermal decomposition temperature was observed for organically-modified layer silicate (OMLS) nanocomposites with polypropylene-graft-maleic anhydride (PP-g-MA) [30,37], PP [38], and polystyrene (PS) [39,40], when compared to their pure polymer counterparts. In particular, thermogravimetric analysis (TGA) experiments performed under N2 showed the onset temperatures for decomposition of polyethylene (PE)/OMLS nanocomposites are approximately 20-30° C. higher than for pure PE [41].
Flammability properties: Cone calorimetry measurements have demonstrated decreased flammability for many types of polymer-clay nanocomposites. The heat release rate (HRR), especially the peak HRR, is an important parameter in evaluating fire safety [42,43]. The reduction in HRR and peak HRR shown by many polymer-clay nanocomposites suggest a decrease in their flammability relative to the pure polymers. Delaminated clay-nylon-6 and -nylon-12 nanocomposites, as well as intercalated clay-PS and -PP nanocomposites, have shown substantial decreases in HRR [44]. Several PP and PP-g-MA nanocomposites also exhibit a reduction in HRR as measured by cone calorimetry [30,37,38,45]. The peak HRR of PE nanocomposites was reduced by 54% [41].
A more severe test of non-flammability is the capacity of a burning material to self-extinguish. Self-extinguishing behavior of PEI-clay nanocomposites has been reported [36], however there is no report of this behavior in polyolefin-clay nanocomposites. Similarly, no report has shown that a polyolefin/clay nanocomposite has achieved a UL94 V0 rating, which is a practical flame retardant material according to the Underwriter's Laboratory's fire test protocol [46].